1. Field
The present application relates generally to wireless communications, and more specifically to systems, methods, and devices for communication in a multiple input, multiple output (MIMO) environment. Certain aspects herein provide for improved utilization of a wireless medium when utilizing MIMO.
2. Background
In many telecommunication systems, communications networks are used to exchange messages among several interacting spatially-separated devices. Networks may be classified according to geographic scope, which could be, for example, a metropolitan area, a local area, or a personal area. Such networks would be designated respectively as a wide area network (WAN), metropolitan area network (MAN), local area network (LAN), or personal area network (PAN). Networks also differ according to the switching/routing technique used to interconnect the various network nodes and devices (e.g. circuit switching vs. packet switching), the type of physical media employed for transmission (e.g. wired vs. wireless), and the set of communication protocols used (e.g. Internet protocol suite, SONET (Synchronous Optical Networking), Ethernet, etc.).
Wireless networks are often preferred when the network elements are mobile and thus have dynamic connectivity needs, or if the network architecture is formed in an ad hoc, rather than fixed, topology. Wireless networks employ intangible physical media in an unguided propagation mode using electromagnetic waves in the radio, microwave, infra-red, optical, etc. frequency bands. Wireless networks advantageously facilitate user mobility and rapid field deployment when compared to fixed wired networks.
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (TDMA) systems, 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, Long Term Evolution Advanced (LTE-A) systems and Orthogonal Frequency Division Multiple Access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-input single-output, multiple-input single-output or a multiple-input multiple-output (MIMO) system.
A MIMO system employs multiple (NO transmit antennas and multiple (Nr) receive antennas for data transmission. A MIMO channel formed by the Nt transmit and Nr receive antennas may be decomposed into Ns independent channels, which are also referred to as spatial channels, where Ns≦min{Nt, Nr}. Each of the Ns independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports both time division duplex (TDD) and frequency division duplex (FDD) operation. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle may allow the estimation of the forward link channel from the reverse link channel. In both FDD and TDD systems, a feedback sent from the user terminal may be used among other methods to convey channel state information to a base station. The channel state information at the base station enables it to extract transmit beamforming gain on the forward link when multiple antennas are available at the base station.